This invention relates generally to the field of data storage devices, and particularly to storage devices that attain both high areal recording density and high mechanical shock resistance. The data storage devices of the invention also have the benefits of mobility and of high-speed recording and retrieval of information, with a much lower consumption of power than required with a traditional hard disk drive (HDD).
Computing and communication platforms are changing and becoming more portable and mobile. The desktop personal computer and other systems such as file servers contain large databases that are not configured to be transportable. A number of devices are available to allow these platforms to communicate with each other, and to share information with mobile units. However, the speed at which data can be transmitted is limited by the bandwidth of these various alternatives. Communication over a telephone line or other hard-wired cable connection can attain impressive transfer speeds, but is not as convenient as a rotating memory device when the amount of information to be transmitted is large. For example a fast modem can transmit data at 53K bits/sec. and an ISDN connection can attain a speed of 128K bits/sec. Rotating memory devices currently operate at about 40,000K bits/sec. and this rate is continuing to improve with advancements in technology.
Newer applications such as digital cameras require a fast, low cost communication channel along with an archival function. Currently these products are designed to be tethered to a personal computer and transfer the images via a cable or utilize very expensive removable semiconductor flash memory cards. The digital images are then stored onto the rotating memory peripheral installed in the personal computer. This storage unit then performs the archival function.
There are generally two types of rotating memory devices. One utilizes a storage medium that is secured and cannot be removed from the device, such as the HDD. The second type utilizes a recording medium that is removable and interchangeable with other similar units, such as a floppy disk drive (FDD). The FDD has the benefit of being the least expensive and most rugged removable medium, while the HDD is typically secured to the desktop computer or organized as a non-transportable disk array in a file server. There are designs that mount a HDD in a drawer that will allow the entire assembly to be exchangeable between various platforms. There are also designs that allow a single hard disk (HD) or a dual hard disk pack contained in a plastic enclosure (cartridge) to be removed and interchanged with another similar disk drive. However, the robustness and reliability of the respective head/disk interface limits the usefulness of these devices.
Hence, it is one objective of the invention to provide a unique recording interface that can resist mechanical shock, that does not compromise storage capacity, data accessing speed or reliability, and which does not increase the production cost of the storage subsystem.
HDD recording heads comprise a recording element or transducer that is mounted at or near the trailing edge of a ceramic body. The ceramic body (slider) is supported by a gimbal arrangement over a thick aluminum or glass disk substrate. During operation, the slider floats on a thin film of air over the spinning disk surface. This film of air provides an air bearing effect, as described in U.S. Pat. Nos. 4,673,996 and 4,870,519, the disclosures of which are herein incorporated by reference. The applied load of the gimbal balances the forces generated in the air bearing film, resulting ideally in a condition of non-contact between slider and disk during full speed disk rotation.
Rigid disks, in use today, range in thickness from about 0.015 inches (15 mils) to about 0.035 inches (35 mils). The slimmer 15 mil thick disks have a smaller outside diameter of about 2.0 inches. The rigid disk presently manufactured in the largest volume has an outside diameter of 3.74 inch (95 mm) and is constructed utilizing a 30 mil thick substrate. These disks operate at speeds of about 4,000 rpm to as high as 10,000 rpm. The spindle motors in these drives consume considerable power as they must accelerate these disks from a stationary position up to operating speeds, and maintain this speed during the record and reproduce operation.
The details of the recording layers on a typical prior art rigid disk 10 are shown in FIG. 1 and consist, first, of a layer of nickel-phosphorous 14 which is plated onto a polished aluminum or glass substrate 12. The entire disk is then polished, and cleaned to attain a smooth surface. A film of chromium 16 is then sputter deposited to a thickness of about 500 .ANG.. A sputter deposition of a magnetic film 18 follows, containing mostly cobalt with certain other elements in small concentrations, with a thickness of approximately 400 .ANG.. The final operation covers the disk with a thin layer of carbon 20 to provide corrosion resistance. The equipment and procedures that deposit these films have been well researched and developed and are being utilized in the manufacture of HDs in very large volumes. All the sputtering operations occur in a vacuum chamber at elevated temperatures of about 250.degree. C. to 300.degree. C. Finally, a topical lubricant is applied to the disk surface that will allow the recording heads to start and stop in contact with the disk surface.
There are certain recording transducer designs that allow a portion of the trailing edge of the recording head slider to drag on the disk surface. The head contact force is kept sufficiently small so as not to wear-off the recording film or damage the recording element. Most HDDs utilize non-contact recording heads which typically maintain a head to disk separation distance of about two micro-inches at the trailing edge of the slider body.
HDD designs require that the recording components be maintained in a clean environment. The magnetic layer on the disk is thin to attain high linear bit density, and the distance between the magnetic film and the recording transducer element is kept small, on the order of two micro-inches. Any contact due to mechanical shock or other disturbance will result in two hard surfaces, namely, the ceramic recording head and the metal disk, impacting each other, creating a local defect or a scratch. Very large shocks can result in catastrophic contact between these surfaces and the generation of wear particles. These particles and the resulting damage to slider and disk surface topography could then restrict the head from ever recovering to its non-contact operating condition. Situations such as these result in reliability problems and the loss of recorded data.
The HDD includes a metallic or glass substrate that is chosen for its isotropic characteristics. The data tracks on the disk are laid out as concentric circles. There is a small-unrecorded band that separates each data track. The width of this band is kept small to maximize the written track width. Typical hard drives currently attain track densities of about 4,800 to 8,000 tracks per inch. A servo system is designed to keep the recording head aligned to the center of the data track and not have it deviate in position by more than 10% of the written track width, resulting in reliable recovery of the recorded information.
Floppy disk drives (FDD) are designed to operate in unsealed environments. The floppy disk is composed of a plastic substrate, such as Mylar, which is coated with a slurry of magnetic particles. FDDs operate at low speeds (typically less than 1000 rpm) with the recording heads sliding on the recording medium. To attain good wear characteristics the magnetic slurry contains a binder and bulk lubricant along with the magnetic particles. This coating process cannot support the same areal density of magnetic recording as sputter deposited films on HDDs. The plastic substrate cannot survive the high temperatures typically required by sputter deposition processes, and consequently, alternative techniques such as vapor deposition and low temperature sputtering have been studied. To date, these techniques have resulted in films with magnetic characteristics that are not significantly better than those of the web/slurry coating process.
There are designs that allow floppy media to operate at high disk speeds, whereby, the flexibility of the disk is utilized to attain certain beneficial characteristics. One such characteristic allows the recording element to fly close to the disk surface, while other portions of the recording head are maintained at higher distances for acceptable reliability in unsealed environments. For example, one such system is described in U.S. Pat. No. 5,675,452, the complete disclosure of which is herein incorporated by reference.
Other methods have utilized standard HDD heads arranged in an opposed configuration on either side of the floppy disk. At high speed, a non-contact condition can be maintained in this arrangement. However, this design relies upon the durability of the floppy media to overcome inevitable tolerance mismatches between the exact location and gimbal conditions of the opposing recording head assemblies. Conditions such as these would result in intermittent contact between the floppy disk and the recording heads.
Another technique utilized a backing plate positioned in close proximity to a spinning floppy disk. The backing plate provided some rigidity to the flexible disk, whereby, a recording head could be penetrated from one side of the disk to attain a non-contact head/disk interface. However, this method can only offer single sided recording.
Magnetic films developed by the web coating process, such as floppy disks, have lower bulk magnetic characteristics and the signal to noise ratio is poorer than that of sputter deposited HDs. Dual coated media such as Fuji's ATOMM floppy disk and vapor deposited media such as Sony's ME also have coating thickness and bulk magnetic characteristics that are quite inferior to HDs. As discussed earlier, floppy product designs require the disk substrate to be compliant, and a plastic film such as Mylar or PET (polyethylene terephthalate) has been successfully used in such applications. These films exhibit anisotropic expansion characteristics, and the servo system of the disk drive has to be designed to correct off-track errors that have frequency components located at twice the rotational speed. Removal and re-installation of the diskette in the disk drive results in disk centering error, whereby, the center of the data tracks recorded on the disk becomes displaced from the center of rotation of the spindle motor. A stationary recording head under these circumstances will see the center of each data track move between two extreme positions during each revolution. The frequency of this movement is at the spindle rotational rate. Anisotropy in the floppy disks causes the data tracks to distort due to changing environmental conditions, and a circular track recorded under one set of environmental conditions can become elliptically shaped under different environmental conditions. A stationary recording head will see the center of the data tracks move between two extreme positions twice in each revolution of the disk, or at a frequency that is two times the spindle rotational rate. The servo control system must correct these positioning errors and must be designed with a much wider band-width than a HDD operating at a similar track density.
In summary, floppy disks have magnetic coatings that have lower signal strength than HDs, requiring a much wider track to attain similar read-back amplitudes. The base material exhibits anisotropic expansions necessitating more stringent servo control. Due to these limitations, a much smaller track density must be utilized in these products. Consequently, it is extremely difficult to develop high storage density, high shock resistant, low power consuming recording apparatus with a floppy disk.
It is therefore an object of the invention to provide a rotating data storage medium which is generally flexible while still providing many of the storage capacity benefits in existing HDDs. In particular, it is an object to provide a flexible data storage medium which is capable of recording and reproducing high areal densities of digital data.